“Mass incarceration on a scale almost unexampled in human history is a fundamental fact of our country today,” writes the New Yorker’s Adam Gopnik. “Over all, there are now more people under ‘correctional supervision’ in America - more than 6 million - than were in the Gulag Archipelago under Stalin at its height.” Is this hyperbole? Here are the facts. The U.S. has 760 prisoners per 100,000 citizens. That’s not just many more than in most other developed countries but seven to 10 times as many. Japan has 63 per 100,000, Germany has 90, France has 96, South Korea has...